1. Field of the Invention
The present invention relates to a semiconductor device having a circuit composed of thin film transistors and to a method of manufacturing the same, for example, an electrooptical device which is represented by a liquid crystal display panel and an electronic equipment on which such an electrooptical device is mounted as a part.
Note that the semiconductor device in this specification denotes devices in general, which can function by utilizing a semiconductor characteristic. The electrooptical device, a semiconductor circuit, and the electric equipment are entirely the semiconductor device.
Also, note that an element substrate in this specification denotes a substrate in general, with which an element utilizing a semiconductor characteristic is provided. As the element, there are, for example, a thin film transistor, an MOS transistor, and a diode.
2. Description of the Related Art
Recently, a technique for constructing a thin film transistor (TFT) using a semiconductor thin film (about several to several hundreds of nm in thickness) formed on a substrate having an insulating surface for has been noted. The thin film transistor is widely applied to an electronic device such as an IC or the electrooptical device. In particular, a development of the thin film transistor as a switching element of a liquid crystal display device is hastened.
In the liquid crystal display device, in order to obtain a high quality image, an active matrix liquid crystal display device in which pixel electrodes are arranged in matrix and the thin film transistor is used as a switching element connected with the respective pixel electrodes has the attention.
In addition, in a display performance of the active matrix liquid crystal display device, it is desired that a pixel has a large retaining capacitance and a high aperture ratio. When each pixel has the high aperture ratio, light utilization efficiency is improved and low power consumption and miniaturization of a display device can be achieved.
Recently, as a pixel size is minute, a high resolution image is desired. When the pixel size is minute, formation areas of the TFT and a wiring, which are occupied in one pixel is expanded, and thus the pixel aperture ratio is decreased.
Therefore, in order to obtain the high aperture ratio in the respective pixels within a specified pixel size, it is necessary to make the layout of circuit elements required for a circuit structure of the pixel with high efficiency.
Further, for a low cost, it is desired that a pixel having a high aperture ratio be realized with a small number of masks.
In addition, in reliability of the active matrix liquid crystal display device, even when it is used for a long period, it is desired that constant display be made without changing an orientation of liquid crystal.
Various studies have been made to achieve the above technique. As a result, the liquid crystal display device having the following structure is realized.
First, in order to realize the liquid crystal display device having a high pixel aperture ratio with a small number of masks, a pixel structure shown in top views of FIGS. 16 and 17 is manufactured. FIG. 17 is a partially enlarged view of FIG. 16. A cross section cut along a dashed line A-A′ in the top view of FIG. 16 is shown in FIG. 9. Cross sections cut along dashed lines C-C′ in the top views of FIGS. 16 and 17 are shown in FIGS. 2A and 2B.
In the structures shown in FIGS. 16 and 17, a first electrode 485 intersects a first semiconductor film 484 through a gate insulating film and has a function as a gate electrode. In addition, the first electrode 485 and a second semiconductor film 493 are used as capacitor electrodes and the gate insulating film is used as dielectric, and thus a retaining capacitor is formed. That is, the first electrode has double functions as the gate electrode and the capacitor electrode. A gate wiring 481 is connected with the first electrode 485 through contact hole.
In addition, a second electrode 492 is connected with the second semiconductor film 493 through a contact hole. The second electrode 492 has a region in which is in contact with a pixel electrode 491. Through the second electrode 492, the second semiconductor film 493 has the same potential as the pixel electrode 491.
Therefore, with respect to the electrodes composing the retaining capacitor, the first electrode connected with the gate wiring through the contact hole has a gate potential and the second semiconductor film connected with the second electrode 492 through the contact hole has the same potential as the pixel electrode 491.
The structures shown in FIGS. 16 and 17 are characterized in that wirings and electrodes in a TFT element, that is, the first electrode (as gate electrode and capacitor electrode) 485, the gate wiring 481, a source wiring 483, and the pixel electrode 491 are formed using three photo masks, and simultaneously the retaining capacitor is also formed using these three photo masks. In addition, since the pixel electrode 491 can be overlapped on the source wiring 483 through an insulating film (not shown), an aperture ratio can be increased. In FIG. 16, with respect to a pixel in a VGA with 43 μm×126 μm, an aperture ratio of 54% is achieved.
However, in the above liquid crystal display device, a phenomenon is observed that the orientation of the liquid crystal located over the first electrode 485 is left after a drive power source is turned off. Thus, there is an anxiety in reliability for a long period. Hereinafter, this phenomenon will be described based on experimental results.
FIGS. 20A to 20C and 21A to 21C show results of orientations in the pixel portion of the liquid crystal display device, which are observed (A) before a drive power source of a transmission liquid crystal display device having the pixel portion shown in FIGS. 16 and 17 is turned on, (B) while a video voltage having ±1 V is applied, and (C) after the drive power source is turned off. FIGS. 20A to 20C show microscope photographs of the liquid crystal orientation (liquid crystal:ZLI4792). FIGS. 21A to 21C show the liquid crystal orientation (liquid crystal:ZLI4792) near the gate wiring 481. The same elements as FIG. 17 are referred to as the same numerals in FIGS. 21A to 21C.
As the liquid crystal in the transmission liquid crystal display device, positive type liquid crystal ZLI4792 produced by Merck Co., Ltd. is used. An orientation film SE7792 produced by Nissan Chemical Industries, Ltd. is used. The orientation of the liquid crystal is a TN mode. When the orientation is observed, the liquid crystal display device is located such that reflection light and transmission light are simultaneously incident into an optical microscope. An optical system of the microscope is adjusted such that a polarization plate is made with crossed Nicols arrangement with respect to both the transmission light and the reflection light. In order to easily observe a change in the orientation of the liquid crystal, a light shielding film is not intendedly provided in a counter substrate.
With a state before the drive power source is turned on (FIGS. 20A and 21A), a specific phenomenon was not observed in the orientation of the liquid crystal over the first electrode 485.
Next, the orientation of the liquid crystal is examined with a state that the video voltage is applied from the source wiring to the pixel electrode through a pixel TFT.
The orientation in the case where the video voltage having ±1 V is applied by a gate inverse drive is shown in FIGS. 20B and 21B. Since a value of the video voltage is equal to or smaller than a threshold value of the liquid crystal, the liquid crystal over the pixel electrode 491 is not switched. Since a gate voltage having ±8 V or −8 V is applied to the liquid crystals over the gate wiring 481 and the first electrode 485, the liquid crystal responds to an electric field and thus oriented such that a major axis of the liquid crystal is in a direction perpendicular to the surface of the substrate. Since the liquid crystal responds to the electric field and perpendicularly oriented, there is a darkly visible region 601 under the polarization plate with the crossed Nicols. The orientation of the liquid crystal responds to the potentials of the pixel electrode, the gate wiring, the first electrode, and the counter electrode. Thus, any specific orientation is not shown.
However, after the drive power source is turned off, the specific phenomenon was observed in the orientation of the liquid crystal.
The orientation of the liquid crystal after the drive power source is turned off is shown in FIGS. 20C and 21C. The orientation of the liquid crystal over the first electrode 485 was fixed and left. In particular, in several tests, even when the drive power source is turned off, there was a tendency that the orientation of the liquid crystal over the retaining capacitor is fixed and left. This region is indicated by reference numeral 602. After the drive power source is turned off, a time until the liquid crystal orientation region 602 is relaxed with the same state as the liquid crystal orientation over the pixel electrode 491 is 10 minutes to 15 minuets.
Next, the reliability of the liquid crystal is tested at a high temperature of 85° C. After a predetermined image is displayed for a long time, the drive power source is turned off and the orientation of the liquid crystal is examined.
Here, results by the reliability test of the liquid crystal are shown. Photographs with respect to the orientation (liquid crystal:ZLI4792) at a room temperature after a lapse of 100 hours in the reliability test are shown in FIGS. 18A to 18C and 19A to 19C. FIGS. 18A to 18c show orientation photographs. FIGS. 19A to 19C show the orientations of the liquid crystal in the pixel portion. The same elements as FIG. 17 are referred to as the same numerals in FIGS. 19A to 19C. The observation is performed at a room temperature.
Even when the drive power source is tuned off after a lapse of 100 hours in the reliability test, the orientation of the liquid crystal over the first electrode 485 was fixed (FIGS. 18A and 19A). A region in that the orientation of the liquid crystal is fixed is indicated by reference numeral 603.
Also, the orientation in the case where the video voltage having ±1 V is applied by a gate inverse drive is shown in FIGS. 18B and 19B. Since a gate voltage having −8 V is applied to the gate wiring 481 and the first electrode 485, the liquid crystals over these respond to an electric field and thus oriented such that a major axis of the liquid crystal is in a direction perpendicular to the surface of the substrate. Since the liquid crystal responds to the electric field and perpendicularly oriented, there is a darkly visible region 604 under the polarization plate with the crossed Nicols.
Next, the orientation of the liquid crystal after the drive power source is turned off is shown in FIGS. 18C and 19C. While the video voltage having ±1 V is applied, a portion of the orientation of the liquid crystal, which is produced over the first electrode 485 is fixed. A region in that the orientation of the liquid crystal is left after the drive power source is turned off is indicated by reference numeral 605.
Thus, even when the drive power source is turned off after the reliability test is performed at high temperature, the orientation of the liquid crystal over the first electrode, that is, over the retaining capacitor is fixed and left.
After the drive power source is turned off, a time until the orientation of the liquid crystal over the first electrode 485 is relaxed with its original state became longer with the progression of the reliability test. After the reliability rest for 1000 hours, a time until the liquid crystal over the entire display screen is uniformly oriented is longer than 1 hour. Even when 1000 hours elapses in the reliability test, an occurrence position of the region in that the orientation of the liquid crystal is fixed is not changed and thus was over the first electrode.
Thus, in the reliability test at a high temperature, there was a tendency that a relaxation time of the liquid crystal after the drive power source is turned off becomes longer. If there is such a result, in particular, in the transmission liquid crystal display device, there is a fear that an orientation relaxation time of the liquid crystal after the drive power source is turned off becomes longer.
It is necessary to remove as an unstable factor a phenomenon that a specific orientation is left after the drive power source is turned off. This is because, if there is such an unstable factor, it is difficult to obtain the long term reliability of the liquid crystal display device.
For commercialization, it is required that such an unstable orientation is covered by providing a light shielding film in a counter substrate. However, when the light shielding film is provided in the counter substrate, an alignment between the counter substrate and an element substrate is shifted to decrease the aperture ratio, and thus there is a possibility that a display quality is deteriorated.
In addition, with respect to a reflection liquid crystal display device, display is performed using external light, and even if the power source of the liquid crystal display device is turned off, display light is incident into eyes. Thus, if it will be a long time before the orientation of the liquid crystal is relaxed, the variations of light and shade by the relaxation process of the liquid crystal are naturally recognized by user's eyes. In many reflection liquid crystal display devices, a light shielding film is not provided in the counter substrate to increase its intensity. Therefore, the relaxation process of the orientation after the drive power source is turned off is easily recognized relatively to the transmission liquid crystal display device.